Many radio frequency (RF) power generators include a controller to regulate the RF output power and prevent damage due to load mismatch, excessive supply voltage, or excessive operating temperature. Such controllers typically include some type of detector, to determine a level of generated power, and a controller, to control the operation of the power generator in response to detected output power or other operating conditions.
At microwave frequencies, it has been a common design approach for quite a number of decades to dispose a circulator between a source of RF power, such as a magnetron vacuum tube, and a system to which the RF power is to be applied. The circulator serves to ensure there is a minimum amount of power reflected back to the magnetron. This provides a more stable operation mode for the magnetron and also has the benefit of enhancing tube life.
At microwave radio frequencies, a so-called ferrite junction type circulator is commonly used. A ferrite junction circulator is formed of a ferrite material within an area joined to three symmetrically placed transmission lines. A permanent magnet produces magnetic flux through the junction. A supplemental electromagnet may be used to control the overall magnetic field applied to the junction. For example, a current applied to the electromagnet can be increased or decreased, in response to measured ambient conditions, to effect overall control of the magnetic field applied to the junction.
Correct operation of a ferrite junction circulator depends on a number of factors, including the inherent characteristics of the ferrite material chosen, the dimensions of the ferrite, and the overall strength of the magnetic field. A problem also exists with maintaining optimum performance of a ferrite junction circulator over a range of temperatures. This is due to a number of factors—one factor being that the intrinsic magnetization of the ferrite material changes with temperature. Thus, determination of an optimized value of magnetization typically requires a different value for the applied magnetic field as temperature changes.
Where both the ferrite material in the junction, and the magnet supplying the applied magnetic field to the junction, are subjected to the same varying temperature, it is possible to cause the magnetic field to vary in a way which compensates for the changes in the ferrite intrinsic magnetization. This is usually done by using a special steel, with rapid changes of its saturation magnetization with temperature, as part of the magnetic circuit.
In applications where the junction is subjected to very large microwave power levels, the ferrite material is subjected to substantial heating. Thus, the problem becomes more complicated. The area of the junction to which the ferrite is attached is normally water cooled to provide for such high power operation. Thus, the ferrite material will be held at a very different temperature than the external magnet(s). Passive magnetic compensation for the changes in the temperature of the ferrite material alone will not therefore produce acceptable results in this application.
One technique for accommodating temperature variation in a water-cooled ferrite circulator has been described by AFT Microwave GmbH of Backnang—Waldrems, Germany. That approach is based on sensing the increase in temperature of the cooling water applied through the circulator. The temperature of this cooling water naturally depends on the amount of power being passed through the junction, and is indicative of the increase in the ferrite temperature. The required adjustment to the magnetic circuit field is then achieved by using control circuitry, driven by measured temperature differentials in the cooling system. This in turn provides an adjustment current to the electromagnet coil, to modify the overall strength of the magnetic field applied to the junction.
This approach of measuring the cooling water temperature has its deficiencies, however. There is a time delay between the heating of the ferrite material and any ultimate final change in its intrinsic magnetization. This results in a delay in the heat being conducted to the cooling water, which of course, drives the input to the control circuit. See Roybal, W. T. “High Powered Test Results at 350 and 700 MHz”, Proceedings of the XX International Linac Conference, Monterey, Calif., pp. 980-982.
In addition, the degree of correction to optimize performance is based on empirically derived data which has been used to set up the control circuit parameters. This cannot be practically done at the customer location on every device delivered. Therefore, normal tolerances on materials and electronics will prevent complete optimization of all units.
For a high power application where the power from the tube may be suddenly increased, the ferrite response time will typically be very short. As the system microwave energy penetrates the ferrite material, junction operation therefore changes rapidly. However, even a rapid change will show immediately in the amount of measured reflected power. Thus, another approach which has been used in the past is to measure an input Voltage Standing Wave Ratio (VSWR) as presented to the magnetron tube. This approach can be used when additional circuit components, such as directional couplers, are available to estimate a forward power Pf and a reverse power Pr. The VSWR can then be estimated by the following calculation:
  VSWR  =            (              1        +                                            P              r                        /                          P              f                                          )              (              1        -                                            P              r                        /                          P              f                                          )      
It is therefore possible to provide a logic system that measures and calculates the VSWR and then controls an electromagnet to adjust the overall magnetic field applied to the circulator. The logic can be arranged to continually minimize the VSWR.
It has also been known, therefore, to provide a detector for measuring forward and reverse power levels at the circulator. These power levels are then digitized and fed to a computing device such as a computer located at a customer site. The computer can be programmed to receive the input measured forward and reverse powers values and determine whether the VSWR is increasing or decreasing. The computer can then also be connected to output incremental changes to a power supply that drives a coil of the electromagnetic circuit, with the computer continuing to make adjustments to the coil current until the measured VSWR is minimized.